1. Field of the Invention
The invention relates to field of disk drives and more particularly to methods for writing servo tracks on magnetic hard disks.
2. Description of the Related Art
Hard disk drives provide prerecorded tracking servo information on the data recording surfaces of their magnetic hard disks. This servo information typically comprises servo bursts spaced evenly along tracks. Data is recorded between the servo bursts. In most cases, servo bursts are radially aligned. This radial alignment makes them look like the spokes of the wheel. When the servo bursts are prerecorded by a linear actuator, the spokes are perfectly straight and follow a radius drawn from approximately the disk's inner diameter to the disk's circumference. If the bursts are written by a rotary actuator, the servo bursts follow the arc of the rotary actuator.
During operation, the disk drive magnetic read/write head flies over the spinning disks and reads information from the tracks as the information passes beneath the head. This information includes both data and servo information recorded in the evenly spaced servo bursts. The servo information tells the disk drive servo system where the head is in relation to the track in order that the disk drive servo system may adjust the head's radial position to keep the head on track center.
The servo bursts may be written onto a disk's surface using a variety of techniques. The most common method is to write the servo onto the disk using the disk drive's own magnetic head controlled typically by an externally introduced picker that grasps the drive's rotary actuator arm upon which the read/write head is mounted. An external mechanism incrementally moves the arm while other circuits command the disk drive to write the servo bursts.
Another common servo-writing method comprises writing servo bursts onto the disks already assembled onto the disk drive spindle but prior to the disk drive spindle/disk combination, also known as a hub/disk assembly (“HDA”), being assembled into the disk drive itself.
A newer approach mounts a large number of disks onto a spinstand and writes the servo bursts onto all of them at the same time. This approach, also known as “multiple disk write” (“MDW”) saves money due to the number of disks that are written at the same time. However, this approach also introduces a number of problems due to the fact that the disks themselves are not assembled onto the disk drive spindle at the time servo tracks are written.
One of the problems is that the disk must be mounted onto the drive spindle in the same “orientation” it was written. For example, if the servo tracks were written by a rotary actuator, the servo burst will be arrayed in a arc that follows the arc of the servo-track writer. In order to read such servo bursts properly, the disk drive's actuator must be aligned to traverse the same arc as the servo track writer. In order that this occur, the disks must be mounted in the same orientation vis-à-vis the disk drive's actuator they were vis-à-vis the servo track writer's actuator. In most cases this is accomplished by assuring that a disk's “top” surface when it is mounted in the servo track writer also be the “top” surface when that same disk is mounted in the disk drive.
A second problem occurs when two (or more) such prerecorded MDW disks are mounted in a single disk drive. Not only must the arcs of both disks be aligned, but also the tracks themselves should be substantially aligned vertically, that is, the tracks of one disk must be substantially congruent with tracks of the other disks such that they form “aligned” cylinders. In this manner, the disk drive can switch between a head reading, for example, track 1000 on the top surface of the topmost disk, to a head reading track 1000 of the top surface of the next disk in the stack without needing to perform a seek. If the respective tracks were significantly misaligned in some fashion, switching between them without performing a seek or some other alignment technique would not be possible. The first thing the drive would need to do after head switch would be to find out its location. This may even require it to seek a track “0” in order to recover a proper track number.
A third problem caused by writing servo data while the disk is mounted on a different spindle is track eccentricity. A hard disk's inner diameter has a tolerance specification much higher than the track eccentricity specification of most disk drive servo systems. If the disk is mounted on the disk drive's spindle in a manner that is significantly off center from the way it was written on the servo writer, it will exceed the drive's servo system eccentricity tolerance. While most disk drive servo systems have eccentricity feed forward mechanisms to help increase their eccentricity tolerances, these feed forward systems have stroke limits far below the highest tolerances for hard disk inner diameters.
Another problem is the angular alignment of the servo spokes from one disk to another. If the angular misalignment is too large, the drive cannot reliably switch heads from one disk surface to another located on a second disk.
A current practice for achieving such alignment is to bias all the disks in the MDW servo-writer against the servo-writer's spindle hub so that each of the disks' inner diameters are vertically aligned at the point of contact with the servo track writer's hub. Thereafter, when these disks are assembled onto the disk drive's spindle, alignment of their inner diameter's contact points that abut against the disk drive spindle hub causes them to have the same vertical alignment they had in the MDW servo track writer. Depending on the precision of alignment of the disks in both the MDW servo track writer and in the disk drive, and upon the precision that the respective servo bursts are written by the MDW servo track writer, the alignment of the inner diameter contact points automatically congruently aligns the tracks of respective disks into the same cylinders they had on the servo-track writer.
The conventional way to align the disks onto both the servo-track writer and disk drive spindles so that the same portion of the disk inner diameter contacts each is to mark the disks with a laser prior to their being loaded onto the MDW servo track writer. The human operator or robot that mounts the disks onto the MDW servo track writer optically inspects the mark and places the disks onto the hub with the mark, for example, being at point where the disk inner diameter abuts the servo track writer hub. The same is done when the disks are mounted on the disk drive hub.
However the use of a laser marking tool presents several problems. The laser marking tools are expensive and bulky. They require substantial clean room space. Different tools are required to mark glass and nickel phosphorus/aluminum substrates. Finally the use of a laser marking tool adversely affects effects drive reliability in a number of ways. The large topographic relief of the marks at the disk inner-diameter can lead head crashes. In addition, the rough topography can lead to the debris generation which can also lead to head crashes.
Thus there is a need to permit multiple disk writers to write the servo bursts of a large number of disks at the same time while not using lasers to mark the disks.